Personally I'd be kind of leary of using a check valve. Any of the ones I have ever seen on systems have all failed after they have been in use for awhile. There's just so much stuff that can grow in the plumbing of a saltwater system and foul them up.
A union check valve that you could take apart and clean would be better as long as you are good about cleaning it on a regular basis.
I think I would use the returns for a closed loop and put a return from the sump over the back with a syphon break on it. Then you would have even more flow,
and less to worry about going wrong.